A STUDY OF BASE CONTACT FORMATION IN EPITAXIAL SI SI0.88GE0.12 HBT STRUCTURES/

An HBT structure was grown by LP-VPE and comprises a 45 nm thick Si0.8 8Ge0.12 base layer, of which the central 25 nm is doped with boron (2 x 10(19) cm(-3)). The base was capped by a 150 nm thick low doped emit ter (LDE) layer (10(18) As cm(-3)). Three different implantation condi tions were employed in order to make contact to the base through the L DE, two of which involved preamorphisation with Ge+. The redistributio n of epitaxial and implanted boron after low temperature (600 degrees C) furnace annealing and rapid thermal processing (RTP) at 1000 degree s C was studied by SIMS. The information from SIMS is correlated with observations of the defect structures (TEM and RBS/C) and amorphous la yer thicknesses before and after annealing. This study shows that prea morphisation leads to a significant reduction in the defect density of the LDE after RTP, reduces the amount of implant channelling and incr eases control over the boron profile. Care must be taken to ensure tha t the residual damage from the implants, does not intersect the base l ayer.